In pregnant women, a normally benign bacterium emits tiny toxic balloons that can cause premature labor and stillbirth, a new study finds.

Called Group B Streptococcus, the bacterium lives in the vaginas of 20 to 30 percent of pregnant women worldwide. Strep B doesn’t cause problems in the lower genital tract. But in pregnant mice, Strep B secretes protein-filled balloons that can travel up into the uterus. Those balloons cause inflammation and weaken the amniotic sac, researchers from India report September 1 in PLOS Pathogens.

Scientists already knew that Strep B can be a problem during pregnancy. They didn’t know that it makes tiny long-range weapons. The danger is “not just the bug alone,” says microbiologist Lakshmi Rajagopal of Seattle Children’s Research Institute, “but also something the bug produces.”

Using a scanning electron microscope

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